Blocked vents prevent air from leaving local pockets between the sheet and tool. The defect usually repeats in the same small area: a soft corner, rounded letter, dull patch, or incomplete recess surrounded by otherwise acceptable detail.
Dust, release-agent residue, plastic pickup, coolant contamination, and fine trimming debris can close vents gradually. A vent may look open at the surface while its connected channel is obstructed below the tool face.
Corrective action
The defect location should be traced to the corresponding vent and internal passage. Cleaning must remove contamination without enlarging the hole or damaging the tool surface. Compressed air should be used only when the debris can be directed safely away from the vacuum system and operator; vacuum extraction is preferable for loose particles.
Recurring blockage calls for filtration, housekeeping, or a tool-design change. Vent diameter and spacing must suit the polymer, sheet temperature, finish requirement, and tool material. Drilling every visible defect is not a reliable fix because poor sealing, trapped geometry, or inadequate manifold flow can produce the same symptom.